The platform stabilizing means comprises an upper extensible, scissors linkage mechanism having upper end portions connected to the platform and a lower extensible, scissors linkage mechanism having lower end portions connected to the base. The lower end portions of the upper extensible, scissors linkage mechanism are connected to upper end portions of the lower extensible, scissors linkage mechanism so that the linkage mechanisms are constrained to move in unison.
Apparatus of the aforementioned type is shown in U.S. Pat. No. 2,725,578. In this known apparatus, several linkage mechanisms form a part of vertically and horizontally extending bracing means. Three separate lazy-tongs are arranged at the sides of an equilateral triangle forming vertical bracing in three mutually inclined vertical planes. Each lazy-tongs is slidably connected to the base and to the platform, and includes upper and lower linkage mechanisms. An intermediate linkage mechanism serves as a connecting means to connect the upper and lower linkage mechanisms.
A central pivot of each of the known three upper, intermediate and lower linkage mechanisms is connected to a peripheral circular ring member at each respective location. The three horizontally disposed ring members form another part of the bracing means for the known lifting apparatus and are secured to the inside of a pleated bellows which serves as a pneumatic thrust mechanism.
In this known apparatus, the three surrounding ring members and the three separate lazy-tongs necessary for platform stabilizatin give rise to various problems, e.g. an extremely high manufacturing cost, complexity, and, most importantly, poor stabilization of the platform due to uneven or unequal movement (extension or contraction) of the lazy-tongs which allows the platform (and/or ring members) to tilt relative to the horizontal. Furthermore, the bearing or pivot clearances are necessary for relative pivotal movement of the parts of the lazy-tongs with respect to each other and to the ring members. Such clearances allow the platform to move linearly in any direction and top the plane of the platform.
In any lifting position of the prior art platform intermediate its maximum upper and minimum lower positions, the bracing means allows the platform to rotate freely through a restricted but unacceptably large angle, thus failing to stabilize the platform and allowing twisting torsion loads to be applied to the relatively weak bellows. Thus, the platform can pitch, roll and yaw to an unacceptable extent.
The known apparatus requires the ring members to be directly attached to the bellows and, thus, subjects the lazy-tongs to lifting forces produced by the bellows. Such structure gives rise to further problems of wear, friction and malfunction of the platform stabilizing means.